Audiology Chapter 9 – The Middle Ear Vocabulary

Learning Objectives

  • 9.1 Explain—in fundamental, yet complete terms—the middle‐ear anatomy & physiology, its embryologic development, and overall purpose.
  • 9.2 List the most prevalent disorders that compromise middle-ear structure or function.
  • 9.3 Identify causes and standard medical/surgical treatments for conductive losses stemming from middle-ear disease.
  • 9.4 Predict classic audiometric patterns (air-bone gaps, tympanograms, reflexes, word scores) produced by each pathology.
  • 9.5 Integrate audiometric + case-history findings to estimate a likely etiology of any given conductive loss.

Overview & Functional Purpose of the Middle Ear

  • Alternate names ▪ “Tympanic cavity”.
  • Principal contents ▪ Ossicles ▪ Stapedius & Tensor-tympani muscles with supporting ligaments ▪ Eustachian tube (auditory tube).
  • Core function ▪ Acts as an impedance-matching transformer that carries airborne vibrations from the external ear to the fluid-filled cochlea.
  • Physical boundaries ▪ Laterally—tympanic membrane (TM) ▪ Medially—oval/round windows + promontory leading into the inner ear.
  • Ventilatory connection ▪ Opens to the nasopharynx through the eustachian tube for pressure equalization.

Development / Embryology

  • Originates—like the outer ear—from the first two pharyngeal arches.
  • Landmarks present by the end of gestational month 2
    • middle-ear space • eustachian tube • oval window.
  • Ossicles begin as cartilaginous models; fully ossified by week 24.

Middle-Ear Cavity: Dimensions & Lining

  • Housed in the petrous portion of temporal bone ▪ Air-filled, oval cavity.
  • Approximate dimensions ▪ ext{Height} \approx 15 \text{ mm}, \text{Width} \approx 6 \text{ mm}, \text{Depth} \approx 6 \text{ mm} (≈ 2\text{–}3 \text{ cm}^3 total volume).
  • Mucous membrane is ciliated → continuous cleansing via mucociliary transport.

Ossicular Chain (Malleus, Incus, Stapes)

  • Smallest bones in the human body; each only 2\text{–}6 \text{ mm} long.
  • Suspended by ligaments but articulate as a lever system that boosts sound intensity to overcome the air-to-fluid impedance mismatch.

Malleus ("hammer")

  • Manubrium (handle) embedded in TM from superior portion to the umbo.
  • Head articulates with body of incus.

Incus ("anvil")

  • Body joins mallear head; lenticular process meets stapes head.

Stapes ("stirrup")

  • Smallest human bone.
  • Head receives lenticular process; footplate anchors in the oval window.
  • Parts: neck, anterior & posterior crura, footplate/base.

Eustachian Tube

  • Course ▪ Anteroinferior passage from middle ear to nasopharynx.
  • Adult characteristics ▪ 30^{\circ} downward angle; length 35\text{–}38 \text{ mm}.
  • Pediatric characteristics ▪ Shorter, wider, more horizontal → predisposes to infection/effusion.
  • Composition ▪ Lateral \tfrac{1}{3} bony; medial \tfrac{2}{3} cartilaginous.
  • Physiologic role ▪ Maintains equal pressure across TM to maximize mobility.
  • Resting state ▪ Closed; opens transiently during yawning, swallowing, sneezing, or Valsalva.
  • Muscular control ▪ CN X (levator veli palatini, salpingopharyngeus) ▪ CN V (tensor veli palatini, tensor tympani).

Mastoid Bone

  • Posterior extension of temporal bone; honey-combed with air cells.
  • External palpation point ▪ Mastoid process—standard placement for bone-conduction oscillator.

Windows & Promontory

  • Oval window ▪ Upper membrane; receives stapes footplate.
  • Round window ▪ Lower, elastic membrane displaces reciprocally to allow perilymph movement.
  • Promontory ▪ Bulge of basal cochlear turn separating the two windows.

Impedance-Matching Mechanisms

  • Needed because fluid impedance > air impedance → unassisted transfer would lose ≈ 28 \text{ dB}.
  • Achieved gain ≈ 30 \text{ dB} via:
    • Area differential ▪ TM effective vibrating area ≈ 17 × larger than oval window ⇒ pressure gain ≈ 23 \text{ dB}.
    • Ossicular lever ▪ Displacement ratio TM : stapes =1.3:1 ⇒ additional ≈ 5 \text{ dB}.

Neural & Muscular Elements

  • Fallopian canal ▪ Houses motor branch of facial nerve (CN VII) across medial wall.
  • Chorda tympani ▪ CN VII branch for anterior \tfrac{2}{3} taste, traverses ME cavity.

Stapedius Muscle

  • Length 7 \text{ mm}; originates posteriorly, inserts on stapes neck.
  • Innervation ▪ CN VII.
  • Acoustic reflex ▪ Contraction pulls stapes laterally → stiffens ossicular chain, attenuating low-frequency energy.

Tensor Tympani Muscle

  • Length 25 \text{ mm}; arises from medial wall, inserts on malleus manubrium.
  • Innervation ▪ CN V (trigeminal).
  • Contraction increases TM tension, implicated in startle & chewing responses.

Step-by-Step Conversion of Energy (“How We Hear”)

  1. Acoustic (air) vibration traverses external canal.
  2. TM sets ossicles into mechanical motion.
  3. Stapes footplate pumps fluids within cochlea (hydraulic energy).
  4. Traveling wave deflects basilar-membrane hair cells.
  5. Hair-cell shearing opens ion channels → electrochemical impulses along CN VIII.
  6. Auditory cortex decodes signal → perception of sound.

General Audiometric Correlates of Middle-Ear Pathology

  • Air-bone gap (ABG) present; BC often normal except where noted.
  • Tympanometry typically abnormal (Type B, C, or As). Static compliance ↓.
  • Acoustic reflexes absent/elevated because ossicular stiffness prevents stapedius contraction from transmitting.
  • Word recognition generally excellent once audibility restored (outer/ME disorders do not distort cochlear/neural coding).

Specific Middle-Ear Disorders

1. Eustachian Tube Dysfunction (ETD)

  • Etiology ▪ Edema from infection/allergy, adenoid hypertrophy, structural blockage.
  • Pathophysiology ▪ Inadequate air replacement → negative ME pressure → TM retraction.
  • Audiology
    • Mild CHL
    • Tympanogram Type C (peak at negative pressure)
  • Management ▪ Treat underlying nasal/allergic cause; auto-inflation; possible PE tubes.

2. Otitis Media (OM)

  • Definition ▪ Infection/inflammation of mucous lining of ME; predominant pediatric illness (≈ 70\% before age 2).
  • Predisposing factors ▪ Immature ET anatomy, barotrauma, dysfunctional cilia, cigarette-smoke exposure.

Duration-Based Categories

  • Acute OM 0\text{–}21 days.
  • Subacute/recurrent 22 \text{ days} – 8 \text{ weeks}.
  • Chronic >8 \text{ weeks}.
  • Dormant ▪ Symptom-free interval before quick recurrence.

Effusion Types

  • Serous ▪ Thin, watery transudate.
  • Suppurative (purulent) ▪ Pus-filled.
  • Mucoid (“glue ear”) ▪ Thick, viscous secretions.
  • Without effusion ▪ Merely negative pressure & TM retraction; ear pain prominent.

Audiometric Picture

  • Flat bilateral CHL; loss magnitude ∝ fluid volume; up to 60 \text{ dB HL}.
  • Tympanogram Type B (flat), reduced static compliance.
  • Reflexes absent.
  • Word recognition excellent at elevated levels.

Treatment / Complications

  • Conservative ▪ Analgesics; limit antibiotics to fever, pain, >72 h infection to minimize resistance.
  • Surgical ▪ Myringotomy → suction fluid → insert PE tube (artificial ET).
  • Untreated risks ▪ Ossicular erosion, TM perforation, mastoiditis, meningitis, toxin-induced high-frequency SNHL.

3. Cholesteatoma

  • Keratinizing, epithelial cyst growing inside ME; often sequel of chronic OME.
  • Epidemiology ▪ Peak 10-19 y; higher incidence in cleft-palate children.
  • Clinical signs ▪ White mass behind TM, progressive CHL, otorrhea (foul discharge), headaches, vertigo.
  • Imaging ▪ CT; audiogram (CHL); tympanometry abnormal.
  • Management ▪ Surgical excision mandatory; antibiotics only temporize.
  • Dangers ▪ Erosive expansion into ossicles, eustachian tube, or cranial vault → brain abscess.

4. Otosclerosis

  • Hereditary spongiotic → sclerotic bone deposition around stapes footplate/oval window.
  • Demographics ▪ Onset puberty–30 y; 70 % familial; 2–3× more common in females; bilateral frequent.
  • Audiometry
    • Early low-frequency CHL progressing to flat CHL
    • Carhart’s notch—pseudo-SN dip at 2000 \text{ Hz} in BC curve.
    • Type As tympanogram (shallow) signalling stiffness.
    • Reflexes absent.
  • Treatment ▪ Hearing aids; stapedectomy with prosthesis—variable success.

5. Mastoiditis

  • Bacterial infection of mastoid air cells, often unresolved OM.
  • Can extend beyond bone → systemic illness; treated via mastoidectomy.
  • Affects mostly children but adults susceptible.

6. Facial Palsy (including Bell’s Palsy)

  • ME infection can erode fallopian canal exposing CN VII.
  • Results in unilateral facial paralysis; Bell’s palsy is idiopathic variant; usually self-resolves.

7. Tympanoplasty / Myringoplasty

  • Reconstruction of TM and/or ossicular chain.
  • Hearing outcome hinges on pre-op ME status & ET function.

8. Patulous Eustachian Tube

  • ET remains permanently open—linked to pregnancy, weight loss, decongestants, TMJ disorders.
  • More common in women.
  • Symptom ▪ Autophony—patient hears own breathing/chewing; “head in a barrel” sensation.

Surgical & Medical Interventions: At-a-Glance

  • Myringotomy + PE tube ▪ Ventilation, pressure equalization.
  • Mastoidectomy ▪ Remove infected cells.
  • Stapedectomy ▪ Replace fixed stapes with prosthesis.
  • Tympanoplasty ▪ Repair TM/ossicles.
  • Pharmacology ▪ Antibiotics (judicious), decongestants, steroids for ETD.

Connections to Previous & Future Topics

  • Outer-Ear module ➔ TM forms lateral ME boundary; disorders (e.g., cerumen) may mimic CHL but with normal tympanogram.
  • Inner-Ear/Sensorineural unit ➔ Pathologies such as chronic OM toxins can secondarily injure cochlear hair cells.
  • Speech audiometry concepts ➔ Middle-ear losses rarely degrade word recognition scores once audibility restored, illustrating the difference between conductive vs sensorineural distortions.

Real-World & Ethical Implications

  • Pediatric OM contributes to language delays; timely screening & treatment essential for educational equity.
  • Over-prescription of antibiotics fosters resistant strains—necessitates evidence-based protocols.
  • Genetic counseling for otosclerosis families.
  • Surgical consent must outline benefits vs permanent CHL risk (e.g., stapedectomy failures, tympanoplasty scarring).

Key Numbers, Ratios & Formulas (All In LaTeX)

  • Area advantage: \frac{A{TM}}{A{OW}} \approx 17:1 \Rightarrow \Delta P \approx 23 \text{ dB}.
  • Lever ratio: \frac{d{malleus}}{d{stapes}} = 1.3:1 \Rightarrow +5 \text{ dB}.
  • Total transformer gain: \approx 30 \text{ dB} (offsets \approx 28 \text{ dB} air→fluid loss).
  • Carhart’s notch frequency: 2000 \text{ Hz}.
  • Eustachian tube length (adult): 35\text{–}38 \text{ mm}; angle 30^{\circ}.
  • Stapedius length: 7 \text{ mm}; Tensor tympani length: 25 \text{ mm}.
  • Maximum CHL from OME: 60 \text{ dB HL}.

Quick Differential Guide (Etiology ↔ Audiogram)

  • ETD → Mild CHL + Type C.
  • Serous/Acute OM → Flat CHL up to 60 \text{ dB} + Type B (normal ECV).
  • Cholesteatoma → Progressive CHL, possibly large ABG; Type B or As; otorrhea.
  • Otosclerosis → Low-freq CHL, Carhart’s notch, Type As; often bilateral.
  • Mastoiditis → Variable CHL; conductive + infection signs; history of OM.
  • Patulous ET → Normal hearing yet autophony; tympanogram may show respiration-linked tracing.

"Need-To-Teach" Take-Home Messages

  • Middle ear transforms acoustic to hydraulic energy with ≈ 30 \text{ dB} gain.
  • Proper ET function is central to ME health; its dysfunction underlies most pediatric OM.
  • Conductive pathologies show air-bone gaps but preserve BC and word recognition.
  • Untreated infections may extend to mastoid, meninges, or cause permanent SNHL.
  • Surgical options exist, but success depends on anatomy, disease extent, and ET ventilation.